Information recording discs such as digital audio or video discs has usually have been produced by a photoresist etching process. At an initial stage of the photoresist etching process, a disc-shaped substrate of, for example, glass having a polished flat major surface is rinsed and thereafter cooled at a prdetermined temperature. About an hour is consumed for these steps. The glass substrate is then coated with a thin layer of photoresist and the resultant laminar structure is prebaked and cooled at predetermined temperatures. Another hour is required for these steps. Upon completion of the cooling step, the light-absorptive photoresist coating of the laminar structure is irradiated with a beam of, for example, laser light which is modulated in intensity in accordance with the signals to be recorded on the disc. About one to two hours are required for this step. The laminar structure having the light-absorptive coating thus processed is then developed, rinsed in water, dried, afterbaked and thereupon further cooled, about an hour being required for these steps. Spiral or coaxial information tracks each consisting of a succession of spaced pits arranged in patterns representative of the pieces of information to be reproduced are formed on the surface of the light-absorptive coating of the photoresist thus processed. A light-reflective coating of metal is then applied to the surface of the light-absorptive coating, about forty five minutes being required for this step. The disc blank now having the light-absorptive and light-reflective coatings on the substrate of glass is transferred to an inspection stage where the disc blank is played back on an experimental basis so as to check for any flaws such as dropouts of signal level. The inspection stage requires a period of about one to two hours. If found acceptable at this stage, the disc blank is transferred to a stage where a stamper from which replicas are to be produced in quantity is to be produced.
A total of four to five hours are thus required from the step of preparing an initial substrate to the step of inspecting the substrate coated with the light-reflective metal coating. This means that an information recording disc can not be evaluated for acceptability for at least about four to five hours after the original substrate is coated with photoresist. If the recording disc produced is found to be unacceptable at the inspection stage, viz., in four to five hours after the original substrate has been coated with the photoresist, another disc must be produced by repeating all the steps above described. This is apparently detrimental to the economy and efficiency of producing an information recording disc. Such a problem results from the fact that the signals recorded on the disc can not be reproduced or monitored immediately after the signals are recorded or while the signals are being recorded on the photoresist coating on the substrate.
The disc produced may also be rejected if the disc, on which the signals to be recorded have been properly recorded, is developed for an unduly long or short period of time. Furthermore, because of the fact that photoresist is sensitive not only to a beam of laser light but to natural light, the laminar structure having the coating of the photoresist cannot be processed in the light. This is also detrimental to the efficient production of an information recording disc. All of these problems encountered in producing an information recording disc basically result from the fact that photoresist is used to form the light-absorptive, information recording coating of the disc.
The signals recorded on the light-absorptive coating on a substrate can be monitored during recording of the signals in a process taught in, for example, U.S. Pat. No. 4,097,895. In the prior-art process therein disclosed, a light-absorptive information recording coating of an organic dye (fluorescein), which is absorptive exclusively to light of a predetermined wavelength, is formed on a light-reflective coating of aluminum preliminarily formed on a disc-shaped substrate of glass. When the recording coating of the disc thus composed is irradiated with a beam from an argon laser, the dye exhibits noticeable absorptivity in response to laser light of a particular wavelength and causes ablation of the coating and produces a pit at the irradiated spot. The information recording coating thus formed with pits is then irradiated with a beam from the argon laser with a power selected to be less than the power of the initial laser radiation. The secondarily radiated laser beam is insufficient in power to be capable of causing ablation of the light-absorptive coating of the dye so that the beam is reflected at those spots where the reflective coating underlying the absorptive coating is exposed through the pits formed by the initial laser radiation. At the undisturbed areas, viz., those areas of the light-absorptive coating where the layer of the dye has not been irradiated with the laser beam, the secondarily radiated laser beam is absorbed and is as a consequence not reflected (or is reflected only to a negligible degree). The presence or absence of a pit, and accordingly the signal recorded on the disc, can be detected through detection of the difference between the luminous energy of the laser beam incident on the exposed light-reflective coating and that of the laser beam incident on the light-sensitive information recording coating.
The process taught in U.S. Pat. No. 4,097,895 is advantageous in that the signals recorded on an information recording disc composed of a substrate and light-reflective and light-absorptive coatings can be monitored during recording thereof, that a wet developing step is not needed, and that the disc with the light-absorptive coating can be processed in the light since the dye forming the coating shows a noticeable absorptivity exclusively in response to a beam of light of a predetermined wavelength. These advantages are, however, offset by the prolonged period of time required for causing the ablation of the dye and the difficulty encountered in forming clear-cut pits in the light-absorptive recording coating. The failure to achieve clear-cut pits in the light-absorptive coating results in an information recording disc with a degraded signal-to-noise ratio.
An improved version of the process taught in the above named patent is disclosed in, for example, Japanese Provisional Patent Publication No. 55-87595 and the article titled "Real-Time Laser Recording Using Dye Vaporization Recording Substance" in Procedings of Institute of Electronics and Communications Engineers of Japan (CMP 79-59) issued on Nov. 22, 1979, Tokyo. In the process disclosed therein, a laser beam of a predetermined wavelength is focused on an information recording disc consisting of a substrate of acryl or a transparent coating of polyester or polyethylene and a coating of the mixture of a dye sensitizer and cellulose nitrate dissolved in a solvent of the ketone family. The dye sensitizer may be Ethyl Red, Methylene Blue or Brilliant Green and the cellulose nitrate used has a degree of polymerization approximating 80. The wavelength of the laser beam to be used with such an information recording disc is selected so that the dye sensitizer exhibits an exceptional absorptivity in response to the particular wavelength.
The use of cellulose nitrate in addition to a dye in forming a light-absorptive information recording coating permits the information recording coating to be sublimated at an increased rate due to the self-oxidation effect of the cellulose nitrate. This process advantageous in that the ablation of the light-absorptive coating proceeds much faster a laser beam of reduced power can be used to form pits in the recording coating (in other words, signals can be recorded on the coating at a reduced temperature, and that the pits formed in the light-absorptive coating have configurations clearer than those of the pits formed in an information recording disc produced in the process originally proposed in U.S. Pat. No. 4,097,895. Problems are, however, still encountered in the advanced process in that the clearness of the configurations of the pits is inferior to that of the pits formed in a light-absorptive coating of photoresist and in that the reaction of the dye and cellulose nitrate with the laser radiation produces considerable quantities of chemical residua on the surface of the light-absorptive coating. The chemical residua thus deposited on the light-absorptive coating impair the signal-to-noise ratio of the resultant information recording disc. By reason of these problems, the process still remains at an experimental stage of research and development and is not acceptable as means to supercede the conventional photoresist etching process when put into practice on a commercial basis.
It is, accordingly, an important object of the present invention to provide an information recording disc which will permit monitoring of the signal during or immediately upon completion of the recording of the signals on a light-absorptive information recording coating.
It is another important object of the present invention to provide an information recording disc which can can be produced without having recourse to the wet developing step which is required in a conventional photoresist etching process.
It is still another important object of the present invention to provide an information recording disc having an excellent signal-to-noise ratio.
It is still another important object of the present invention to provide an information recording disc having a light-absorptive information recording coating formed with satisfactorily clear-cut pits.
It is still another important object of the present invention to provide an information recording disc which can be produced with practically negligible quantities of chemical residua produced on the surface of the light-absorptive coating by irradiation of the information recording coating of the disc with a recording beam of light.
It is, yet, another important object of the present invention to provide a process of producing an information recording disc of the above described nature.